As you go through the home office expense section of your return, look for the question near the end of the section that says, "Did you have any home office expenses not deducted in a prior year?' Answering yes to this question will open the page for entering prior year expenses that can carry over.
In 2019 I rented an office. I have carryover from 2018 Form 8829. Where do I put it on the 2019 return? If I don't add information to the requested information in "Home Office" it won't show me the "Did you have any home office expenses not deducted in a prior year?" question.
If you don't have a home office in 2019 then there is no where to enter the 2018 carryovers unless you use the FORMS mode to bring up the unused form and do some direct entries.
Do I use Form 8829 or the "Other Income" on Form 1040 Schedule 1 line 21 with a negative?
It is NOT other income ... do NOT use a negative amount on that line ... doing so will insure an IRS audit. The loss carryover from the 8829 must remain on the 8829. If you are not using the 8829 this year then there is nothing to enter on this year's return ... retain the old 8829 for future reference.